1. Field of the Invention
This invention relates to a process for the production of a lower carboxylic acid anhydride from the corresponding mono-acyl fluoride.
2. Description of the Prior Art
It is known that olefins react readily with carbon monoxide at low temperatures and under moderately high pressure in the presence of a strong acidic medium, typically representative of which include anhydrous hydrogen fluoride, concentrated sulfuric acid and anhydrous chlorosulfonic acid. The postulated carbonylation mechanism involved is conventionally referred to as the Koch reaction and is exemplified in U.S. Pat. No. 2,831,877. The foremost problem faced in any commercial implementation of the Koch reaction in the production of a lower carboxylic acid resides in the difficulty experienced in recovering the acid catalyst in the form suitable for recycling.
Komatsu et al in Bull, Jap. Pet. Inst., 16 124-131 (1974) address this recovery aspect as such specifically applies to the Koch reaction employing a lower olefin. A procedure is accordingly outlined therein for the recovery of HF following preparation of pivalic acid in accordance with the Koch reaction. Essentially the procedure involves hydrolyzing the carbonylation reaction mixture with an equimolar mixture of hydrogen fluoride and water (54% aqueous HF) and distilling to recover anhydrous HF. Thereupon the residue in the form of a equimolar HF complex of pivalic acid is hydrolyzed using a minimum amount of water to provide two layers; the top layer being the carboxylic acid product and the other being recyclable 54% aqueous HF.
Unfortunately, this method is not applicable in a like process for preparing isobutyric acid insofar as the said carboxylic acid is soluble in 50% aqueous HF. The only way to effect phase separation is to dilute the carboxylic acid solution with a substantial amount of water. The net result is that a mole of HF per mole of the carboxylic acid product is effectively non-recoverable for use in the process.